Devices for sensing an average temperature of a fluid within a container, particularly those relying upon electrical bridge networks to integrate several simultaneous signals, are well known in the instrumentation art. Such devices are usually constructed with one or more electrical temperature sensors located in the container and directly exposed to the fluid. In one such device, the temperature sensor is mounted within a refrigerator cabinet and directly exposed by vent openings to refrigerated air circulating within the cabinet. In another device, several temperature sensitive resistance coils of different lengths are vertically positioned within an oil tank and selectively connected to a bridge circuit to provide a measurement indicative of the average oil temperature over the depth of the tank. In effect, these devices measure the instantaneous average temperature over the volume of fluid and not the average temperature of the fluid over a period of time. Temperature measurements provided by these devices vary instantaneously with changes in the temperature of the fluid because of the direct exposure of the sensor to the fluid. The measurements, therefore, respond to short-term fluctuations in temperature due to convection, fluid circulation, loss of heat due to radiation, and cyclical operation of the container's cooling or heating plant, as well as to longer term changes in temperature due to factors such as diurnal absorption of ambient heat.
Occasionally, it becomes necessary to measure the period and amplitude of only the longer term fluctuations in the temperature of a large fluid mass for such purposes as gauging the performance of a process exposed to the mass, for calibration of a temperature sensitive apparatus, or for estimating future cooling and heating needs. Longer term fluctuations may have an amplitude of only one or two degrees over a period of one or more hours while short-term fluctuations usually have periods of less than five minutes. To measure the period and amplitude of the longer term fluctuations, therefore, it is necessary to suppress the effects of cyclical and short-term fluctuations in temperature by continuously averaging those fluctuations over the longer period. The instantaneous responsivity of prior art devices to short-term fluctuations renders them unsuitable for making these measurements, particularly if the fluid is sometimes in motion and at other times still. Moreover, without prior knowledge of the period and amplitude of the longer term fluctuations, periodic sampling of a fluid temperature is an unsuitable alternative method of making these measurements because it is likely to yield misleading data, particularly if the periods and amplitudes of the short-term fluctuations are also unknown. Another alternative, continuous sampling, is also unsatisfactory because it will yield an unnecessary profusion of data while tying-up expensive instrumentation.